Magnetite particles and maghemite particles are generally used as electromagnetic wave absorbing material, electromagnetic wave shielding material or their base. Electromagnetic wave absorption or shielding is effected by coating an apparatus, etc., which is the source of generation of electromagnetic waves, with a coating material prepared by dispersing and mixing magnetite particles or maghemite particles in a vehicle.
Magnetite and maghemite particles are also widely used as a magnetic material for magnetic recording. Magnetic recording media such as magnetic tapes and magnetic discs are produced by coating the tapes or discs with a magnetic paint obtained by mixing magnetic particles such as magnetite and maghemite particles with a vehicle.
Further, because of their particular coloration, magnetite particles being black while maghemite particles assuming a brown color, these particles are commonly used as a pigment for paint in preparation of paint by mixing said particles with a vehicle. These particles are also used as a colorant by mixing them in rubber or plastic materials.
As described above, magnetite particles and maghemite particles are now used in the various fields of industry. In whatever field they may be used, it is the common requirements for these magnetite and maghemite particles to have the following characteristics: easy to work into a paint; capable of being filled to a high density in vehicles or resins; readily and well dispersible and orientable; and capable of effectuating a high degree of contact between the particles.
These requirements are mentioned, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 104923/80 discloses that "There takes place a very remarkable parallel orientation of individual particles in the coating material . . . This enables the particles to be filled to an extremely high density, making it possible to realize an increased anticorrosive effect, more effective shielding against the field of electromagnetic interference and elevated electroconductivity", and Japanese Patent Application Laid-Open (KOKAI) No. 28700/76 discloses that "The magnetic powder used in this invention is characterized by the fact that it can maintain a well satisfactory coating quality even if the packing density of the powder in organic binders is raised . . . Drastic improvement of packing density of said magnetic powder has enabled it to have a high magnetic flux density." A relating mention is also made in PETROTECH, Vol. 9, No. 6 (1986) which discloses on page 494 that "This is classified into the field of material technology for electromagnetic wave shielding . . . It is a conductive coating method which is most popularly used at present . . . Fine particles of nickel, etc., are mixed in the paint . . . Contact between metals is essential . . . Particles with a high degree of contact are selected. . ."
For meeting the above characteristic requirements, magnetite and maghemite particles must be a plate-like fine particles.
This fact is mentioned in many patent applications. For instance, aforementioned Japanese Patent Application Laid-Open (KOKAI) No. 28700/76 discloses that "It is intended to provide a uniform magnetic coating having a high degree of packing of magnetic powder and excellent magnetic properties by coating an essentially plate-like magnetic powder . . . " Also, Japanese Patent Application Laid-Open (KOKAI) No. 104923/80 discloses that "There are other uses of hexagonal flaky (plate-like) iron oxide having a magnetite or maghemite structure . . . There takes place a very remarkable parallel orientation (orientability) of individual particles . . . This enables the particles to be filled to an extremely high density . . . Japanese Patent Application Laid-Open (KOKAI) No. 266311/86 discloses that "Use of fine plate-like ferromagnetic powder of cobalt-containing iron oxide having a particle size of not more than 1 .mu.m would make it possible to provide magnetic recording media having good dispersibility and filling characteristics of powder and excellent surface smoothness."
The following methods are known for producing the plate-like magnetite particles: the method in which an alkaline suspension containing ferric hydroxide or goethite is autoclaved to produce plate-like hematite particles from the aqueous solution and these plate-like hematite particles are reduced under heating in a reducing gas; and the method in which by rapidly oxidizing ferrous hydroxide contained in an alkaline suspension with a strong oxidizing agent, or by reacting a ferric salt and an alkali in an aqueous medium in the presence of a specific additive to form ferric hydroxide, then this ferric hydroxide is autoclaved, plate-like goethite particles are produced from the aqueous solution and these plate-like goethite particles are dehydrated by heating and then reduced under heating in a reducing gas.
In the former method are included the processes disclosed in aforementioned Japanese Patent Application Laid-Open (KOKAI) Nos. 28700/76 and 104923/80. The processes proposed in aforementioned Japanese Patent Application Laid-Open (KOKAI) Nos. 266311/86 and 104923/80 are included in the example of the latter method.
As the method for producing plate-like maghemite particles, there are known the method in which an alkaline suspension containing ferric hydroxide or goethite is autoclaved to produce the plate-like hematite particles from the aqueous solution, and these plate-like hematite particles are reduced under heating in a reducing gas and then oxidized under heating; and the method in which by rapidly oxidizing ferrous hydroxide contained in an alkaline suspension with a strong oxidizing agent, or by reacting a ferric salt and an alkali in an aqueous medium in the presence of a specific additive to form ferric hydroxide, then this ferric hydroxide is autoclaved, plate-like goethite particles are produced from the aqueous solution, and these plate-like goethite particles are dehydrated by heating, then reduced under heating in a reducing gas and oxidized under heating.
As the example of the former method, there are the processes disclosed in aforementioned Japanese Patent Application Laid-Open (KOKAI) Nos. 28700/76 and 104923/80, and included in the latter method are the processes shown in aforementioned Japanese Patent Application Laid-Open (KOKAI) Nos. 266311/86 and 104923/80.
Plate-like magnetite particles and plate-like maghemite particles capable of high packing density and having excellent dispersibility and orientability are most ardently required at present. In production of such magnetite and maghemite particles according to the known methods as mentioned above, there tends to take place sintering of or between the particles due to the rapid growth of unit particles when the plate-like hematite particles produced from the aqueous solution are reduced under heating in a reducing gas. Consequently, it becomes difficult to disperse the particles in a vehicle or resin, resulting in a reduced packing density and poor orientability. As is well known, in the step of heating and oxidizing plate-like magnetite particles to form plate-like maghemite particles, there takes place no growth of unit particles and accordingly no sinter of or between the particles occurs.
Further, in case of using the former type of method described above, it is difficult to produce the fine plate-like hematite particles having an average diameter of not more than 1 .mu.m, especially not more than 0.5 .mu.m, from the aqueous solution. Naturally, it is difficult to obtain plate-like maghemite particles having an average diameter of plate surface of not more than 1 .mu.m, especially not more than 0.5 .mu.m by reducing said hematite particles under heating and to obtain plate-like maghemite particles having an average diameter of plate surface of not more than 1 .mu.m, especially not more than 0.5 .mu.m by oxidizing the resulting plate-like magnetite particles. This fact is seen from, for instance, the statement in aforementioned Japanese Patent Application Laid-Open (KOKAI) No. 28700/76 which discloses that "Hexagonal plate-like hematite (.alpha.-Fe.sub.2 O.sub.3) occurs in nature as micaceous iron oxide and has been known and used as an inorganic anti-corrosive paint. Recently, it has become possible to artificially synthesize this compound, . . . Such synthetic iron oxide is about 1 to 40 .mu. in plate diameter . . . "
In case of using the latter type of method described above, the obtained plate-like magnetite and maghemite particles have many pores on the particle surface and in the inside of particle as a result of dehydration in the goethite crystal particles when they are heated. When such porous plate-like magnetite or maghemite particles are dispersed in a vehicle or resin, the magnetically polarized surface portions attract other fine particles, causing aggregation of many particles to form aggregates of fairly large sizes. This makes it difficult to effect desired dispersion, resulting in a reduced packing density and poor orientability.
As understood from the foregoing statement, the establishment of a process capable of directly producing fine plate-like magnetite particles from an aqueous solution has been strongly desired for obtaining the poreless and non-sintered fine plate-like magnetite and maghemite particles.
Extensive studies have been made by the present inventors in search of such process, and as a result, the process of the present invention has been accomplished.